It is not the accessory plug 54 I have seen the one you mean mine has not got it just a blank whole where yours is.
The plug you are looking at is a Male and looks like the opposite to the Accessory plug which is Female.
This is the plug your looking at
And this is the Accessory plug

But I am not sure what that plug is for looking at it, it is connected to the OBD plug the main OBD is plug M40 I think and this plug is M41